Conventional seafloor electromagnetic prospecting systems have been devised and have been used both by academics and commercial organizations for underwater exploration. One common system consists of a transmitter electric dipole towed behind a ship near an array of receiver electric field dipoles, which are dropped individually to the seafloor and subsequently recovered.
This conventional system is limited in a number of ways. Data is available only after recovery of the receivers. Knowledge of the orientations of the receiver antennas is limited by accuracy of compass data and relies on the antenna arms falling in an orthogonal cross-like pattern, which is difficult to precisely control. Further, the estimates of the time of travel of an electromagnetic signal or equivalently the phase of a single transmitted frequency are limited by the stability of clocks. Each instrument has to have its own clock, which although synchronized against a master clock at deployment, is prone to drifting as the survey proceeds through the day introducing error. Further, the distance between the transmitter and a receiver has to be obtained by active acoustical systems with limited resolution so that the distance between the transmitter and a receiver is also determined erroneously. Receiver deployment and recovery is a time consuming process and some receivers are lost due to release failures.
It would be desirable to have a system for seafloor electromagnetic prospecting that addresses at least some of the shortcomings of the conventional systems.